The term “rapid prototyping” (RP) covers generative manufacturing processes in which 3-dimensional models or components are prepared from computer-aided design data (CAD data) (A. Gebhardt, Vision of Rapid Prototyping, Ber. DGK 83 (2006) 7-12). These are processes such as e.g. stereolithography (SL), selective laser sintering (SLS), 3-D printing, fused deposition modelling (FDM), ink-jet printing (IJP), 3D plotting, multi-jet modelling (MJM), solid freeform fabrication (SFF), laminated object manufacturing (LOM), laser powder forming (LPF) and direct ceramic jet printing (DCJP), with which models, components or spacers can be prepared cheaply even on a small scale (A. Gebhardt, Generative Fertigungsverfahren, 3rd ed., Carl Hanser Verlag, Munich 2007, 77 et seq.). Stereolithography involves RP processes (A. Beil, Fertigung von Mikro-Bauteilen mittels Stereolithographie, Düsseldorf 2002, VDI-Verlag 3 et seq.) in which a spacer is constructed in layers from a liquid and curable monomer resin on the basis of CAD data.
Stereolithographic processes for the preparation of dental mouldings such as inlays, crowns or bridges are highly advantageous particularly with ceramic materials because the impression-taking and casting processes and the grinding and milling operations respectively, which involve considerable manual outlay in the dental engineering laboratory, can thus be greatly simplified and at the same time the large material loss which occurs with non-generative processes can be avoided. As a complete digital process chain is in place today, the standard process steps for the preparation of e.g. multi-unit bridge frameworks (alignment in the articulator, wax modulation, embedding and casting) can be replaced by the digitalization of the model, virtual design of the dental spacer and its generative stereolithographic manufacture.
In the stereolithographic preparation of ceramic spacers a ceramic green compact is firstly prepared by layered radiation curing of a free-flowing ceramic slip which is then sintered after debinding to form a dense ceramic moulding. The green compact is also called a green body. The term debinding is used to describe the elimination of the binder. Here, the binder used is usually removed by heating the green compact to a temperature of approx. 90° C. to 600° C. It is essential that the formation of cracks and deformations is very largely avoided. The green compact becomes the so-called white body as a result of the debinding.
In debinding, purely thermal and thermochemical processes take place. Mixtures of water, solvents, polymers, waxes or oils are usually used as binders in the pressing of ceramic powders. Polypropylene, polyethylene, polyvinyl acetate, polyvinyl alcohol, methylcellulose, polyvinylpyrrolidone, polystyrene or polyethyl methacrylate are mostly used as polymers (cf. R. Moreno, Amer. Cer. Soc. Bull. 71 (1992) 1647-1657). These are linear polymers which are broken down more or less easily into volatile components by depolymerization or chain-splitting at increased temperature.
In the case of green bodies produced by stereolithography based on cross-linking monomer mixtures, there is a polymer network. Through the use of cross-linking monomers the curing time which is required to obtain a stable solid can be significantly shortened, but at the same time the polymer network that forms also displays a much higher thermal stability compared with linear polymers, which adversely affects the debinding process.
The sintering of the white body takes place in the sintering furnace during high-temperature firing. The finely-dispersed ceramic powder is compacted and solidified by exposure to temperature below the melting temperature of the main component, as a result of which the porous component becomes smaller and its strength increases.
EP 1 021 997 A2 describes the use of the laser-sintering process for the preparation of dental restorations. Here, metal powders are sintered in layers using a laser.
DE 101 14 290 A1 relates to the preparation of dental spacers by 3-D plotting using materials that are meltable, condensable, curable thermally or with UV or visible light, filled or unfilled. Proposed for the preparation of green bodies are inorganic pastes which are composed of glass, glass ceramic or ceramic powder, which is converted into a shapable paste with solvent, binder and plasticizer. The powders used are not surface-modified.
WO 97/29901 describes a process and an apparatus for the preparation of 3-dimensional components of a liquid, curable medium. The component is constructed in layers by scanning each individual layer with a laser and curing it. The next layer of the curable material is then deposited by means of a coating device and then likewise cured.
A stereolithographic process for the preparation of dental implants is known from WO 95/28688.
U.S. Pat. No. 5,496,682 discloses light-curable compositions for the preparation of three-dimensional bodies by stereolithography, which contain 40 to 70 vol.-% ceramic or metal particles, 10 to 35 wt.-% monomer, 1 to 10 wt.-% photoinitiator, 1 to 10 wt.-% dispersant and preferably also solvent, plasticizer and coupling agent.
U.S. Pat. No. 6,117,612 discloses resins for the stereolithographic preparation of sintered ceramic or metal parts. The resins have a viscosity of less than 3000 mPa·s. For their preparation, monomers with a low viscosity are used, preferably in aqueous solution. A high solids content and low viscosity are said to be achieved through the use of dispersants.
DE 10 2005 058 116 A1 discloses suspensions for the stereolithographic preparation of ceramic implants in the manner described in U.S. Pat. No. 6,117,612, which do not contain diluents such as water or organic solvents, as the latter are said to increase the viscosity through local evaporation when energy is introduced. The viscosity of the suspension is set at less than 20 Pa·s by varying the concentration of a dispersant. Alkyl ammonium salts of copolymers with acid groups are used as dispersants, wherein the latter can also be coated onto the particles of the ceramic powder.
Methods and compositions for the stereolithographic preparation of ceramic components are described in US 2005/0090575 A1. It is stated that spacers prepared with the liquid materials known from U.S. Pat. No. 5,496,682 are soft and therefore require an additional curing step in order to avoid deformations during firing, while mouldings obtained from paste-like materials form internal stresses during debinding which lead to cracks during sintering. To avoid these problems, plasticizers are used and the quantity of ceramic powder chosen such that the viscosity of the compositions is at least 10,000 Pa·s.
Compositions curable with visible light and their use for the preparation of dental restorations from plastic materials with RP processes are described in DE 199 38 463 A1 and DE 199 50 284 A1.
In RP processes, the composition and the properties of the radiation-curable slip are of decisive importance. Thus as high as possible a volume fraction of the ceramic particles in the slip is required in particular for a high density and final strength as well as good accuracy of fit of the ceramic spacer. Furthermore, a well-set rheology of the slip is a basic requirement for the stereolithographic construction of a defect-free green body, wherein the viscosity and the flow behaviour depend among other things on the size and the content of the ceramic particles in the slip. It is also to be borne in mind that the binder can be removed residue-free without the formation of cracks or stresses during the debinding of the green body.
It is furthermore important that the slips used for the preparation of the green compacts are storage-stable over a sufficient period of time and are largely inert vis-à-vis the tank material, the base surface and the other components with which it comes into contact in the course of the stereolithography process.
A particular problem in the preparation of ceramic spacers by RP processes is the colouring of the ceramic, as the colorants used must survive the debinding and sintering process.
The known slips are not satisfactory in respect of the abovenamed requirements. The object of the invention is therefore to provide improved light-curing slips for the stereolithographic preparation of ceramic and glass ceramic spacers which satisfy the above requirements. The slips are intended to produce green compacts with sufficient strength which can be debound without deformation, the formation of cracks or stresses and which produce upon sintering high-strength ceramics which are suitable for dental purposes. According to a preferred embodiment, the ceramics are to have a colouring adapted to the desired intended use.